JRiver- I have to agree with Mono. If she can look deep down into herself and say "This is worth it. I can do this." great. But, if not, or if she's "settling" as he puts it, she needs to move on.
You talk alot about how "conditioning" is why people feel they need to be mono and how it is "false" and can be "overcome". But it isn't false. And it isn't always about conditioning. Sometimes....people are just mono. Because that's who they are. Just as they are poly, gay, straight, bi, etc. And we can love more than one person, yes, but that love takes different forms. Maybe only one love can be romantic.

Distraught- It seems as if, if you decide to stick with him, you will have to share him. It doesn't mean he loves you less because he loves his wife. It doesn't mean that, given time and effort, you can't establish a more friendly and respectful relationship with his wife. And it doesn't mean your relationship with him is threatened-at least not in his eyes. What you have to do is basically what Nyx suggested. Take a good look at yourself, your emotions, your thoughts, and your relationship with him. Then decide if what you gain from it is worth what you feel you're losing. All relationships are compromise. But none should feel one sided or settled for.